Facts About Troposphere and Tropopause

Troposphere is the lowest portion of Earth’s atmosphere and contains approximately 80% of the atmosphere’s mass and 99% of its water vapour and aerosols. The average depth of the troposphere is approximately 17 km in the middle latitudes. The characteristic features of the Troposphere are its great density. In addition to nitrogen and oxygen, carbon dioxide, and water vapour (nearly all of the water vapour contained in the atmosphere is concentrated in the troposphere) and of numerous particles of various origin.

Thickness of Troposphere

It thickness of the Troposphere  is maximum at equator, deeper in the tropics, up to 20 km , and shallower near the polar regions, at 7 km  in summer, and indistinct in winter. In India, it is taken to be around 16 Kilometers.  The thickness of the troposphere and consequently the atmosphere  is maximum at the equator due to the reasons discussed below:

High insolation and strong convection currents occur over the Equator

One of the laws of Ideal gases called Charles’ law says that in an ideal gas, density decreases with increasing temperature, when pressure is constant. The hot air rises and the Earth is not equally heated everywhere. The troposphere is thicker over the equator than the poles because the equator is warmer. Heat differential on the planet’s surface causes convection currents to flow from the equator to the poles. This implies that the warmer the weather, the thicker is the troposphere. Thus the simple reason is thermal expansion of the atmosphere at the equator and thermal contraction near the poles.

Air is less dense at Equator

Over equatorial regions, where the surface is being heated strongly throughout the year and air warmed by contact with it is expanding and rising, the air all the way up to the tropopause is less dense than air to the north and south. Thus, density of the air is maximum at the equator. But here, you must note that almost same amount of atmospheric mass exists at both equator and poles but only the density of the air is less at equator and greater at poles

Poles Exert more gravitational pull on atmospheric gases

Gravity increases from equator to poles as the earth is not a perfect sphere. That means the gravitational force is more over poles. Hence the atmosphere is pulled with more force near the poles and leads to contraction of the atmosphere.

The centrifugal force due to Earth’s rotation is maximum at Equator

Because the speed of the rotating earth is greatest at the equator the atmosphere tends to bulge out due to friction and Coriolis force.

Chemical Composition of Troposphere

The chemical composition of the troposphere is essentially uniform, with the notable exception of water vapour. The amount of water vapour decreases strongly with altitude. Thus the proportion of water vapour is normally greatest near the surface and decreases with height.

Temperature of Troposphere

Temperature of the troposphere decreases with height. The rate at which the temperature decreases is called the Environmental Lapse Rate (ELR). The environmental lapse-rate (ELR) is about 0.6°C per every 100 meters. Temperature decreases at a nearly uniform rate with increased altitude.

The reason for lapse is that maximum absorption of the sun’s energy occurs at the ground which heats the lower levels of the atmosphere, and the radiation of heat occurs at the top of the atmosphere cooling the earth, this process maintaining the overall heat balance of the earth.

Tropopause

The boundary between troposphere and stratosphere, called the tropopause, is a temperature inversion.  Tropopause refers to the altitude at which the fall in the temperature is stalled. This layer separates the troposphere from the stratosphere (the second layer of the atmosphere). This layer is usually quiet and no major movement of air takes place in it. Its height at Tropic of Cancer and Tropic of Capricorn is roughly 10 to 15 km, highest at the equator 18 km and at the poles it is about 8 km above the earth. In India, the tropopause is generally at a height of around 16 km. The altitude of the tropopause varies with the variations of sea — surface temperature, season, latitude, and weather systems, such as the passage of cyclones and anti-cyclones. So, Tropopause is not a hard lined boundary. The higher is the temperature  of the lower layers, the higher  is the  height of this layer, the layer is lower where there is a cyclone below it. Also note that the tops of cumulus-nimbus clouds often float in his region.